Public Release: 

First glimpse inside cold antimatter atoms

American Institute of Physics

For the first time scientists have been able to peer inside an atom made entirely of antimatter, to get a glimpse of its internal structure. The ATRAP Collaboration of scientists (from Harvard University, the Forschungszentrum Jülich, CERN, the Max-Planck-Institut für Quantenoptik in Garching and the Ludwig-Maximilians-Universität München, and York University) work at CERN. This collaboration includes scientists who first observed high velocity antihydrogen atoms, who developed the techniques for accumulating cold antiprotons, and who have made the most accurate studies of hydrogen atoms.

ATRAP uses antiprotons from CERN's Antiproton Decelerator, and positrons from a radioactive source, to produce cold antihydrogen. The antiprotons are dramatically slowed and cooled, then accumulated using techniques developed by ATRAP and its predecessor. The positrons are slowed, cooled and accumulated using techniques developed by ATRAP members. The antihydrogen forms in a nested Penning trap, a device developed by ATRAP scientists to allow the gentle collisions of antiprotons and positrons needed to form cold antihydrogen.

The new method used by ATRAP to detect the antihydrogen atoms provides a signal only in response to an antihydrogen atom - there is never a background of false signals. ATRAP is now able to detect more antihydrogen atoms in an hour than the sum of all antimatter atoms ever reported. The paper refers to actual observations of a sample of more than 1400 cold antihydrogen atoms.

With substantial numbers of antihydrogen atoms there is hope that eventually enough atoms will be created to allow lasers to probe for any tiny differences between antihydrogen and hydrogen atoms. Such measurements would test fundamental theories of physics, and might even provide some information about the mystery of why our universe is made of matter rather than antimatter. With cold antihydrogen atoms, whose temperatures are within a few degrees of absolute zero, the scientists hope to eventually be able to use special magnets to capture the precious atoms for the precise studies. The detected atoms are nearly cold enough to be captured, though no trapping of antimatter atoms has yet been attempted.

Antihydrogen atoms are the simplest of antimatter atoms. Hydrogen, the simplest matter atom, has an electron in orbit about a proton. Replacing the proton with its antimatter counterpart, the antiproton, and the electron with its antimatter counterpart, the positron, would change hydrogen to antihydrogen. The particles and the antiparticles have the same mass, and the same amount of charge, but opposite sign of charge. When a particle and its antiparticle collide they "annihilate" - both disappear and release energy. Current physics theories predict that the antihydrogen and hydrogen atom would have the same properties.

If an antihydrogen atom is put near a battery, the positive charge of its positron is attracted towards the negative terminal of the battery, while the negative charge of its antiproton is attracted to the positive terminal of the battery. If the battery has a high enough voltage, the strain on the atom will pull the atom apart. If the positron and antiproton are far apart in the atom, then a very small voltage will pull the atom apart. If they are closer together, more voltage will be required to disassemble the antimatter atom. This is the basic idea used by ATRAP scientists to probe the antihydrogen atom. They are able to get a first glimpse of the atom's states, that is, about how closely the antiproton and positron are spaced, by seeing which voltages applied within their apparatus cause the antihydrogen to come apart.

The ATRAP scientists avoid any false signals of antihydrogen because when they take apart an antihydrogen atom, they capture the antiproton in a device called a Penning trap. They then hold the antiprotons as long as they wish, until after all the noise associated with the collisions that form antihydrogen has died away. These antiprotons are then allowed to collide with matter, whereupon their annihilation causes flashes of light in surrounding detectors that can be easily and reliably be counted. In other experiments, there are often false noise signals generated that cannot be distinguished from real signals. Even if the average number of false signals can be estimated for such experiments, one never knows for sure which individual signal is real. The ATRAP scientists are quite sure that the antihydrogen atoms are created when two positrons collide with one antiproton in a process called "three body recombination", in part because they had predicted that this process would produce antihydrogen atoms at a high rate. They believe that the rate is likely increased because they use the lowest temperature and best vacuum ever used for such experiments.

In a second paper (submitted to Physical Review letters and now being considered for publication), ATRAP reports an even more efficient method for producing antihydrogen, in which antiprotons are driven into repeated collisions with cold positrons. The production rate is high enough that for the first time a distribution of antihydrogen states is measured.

ATRAP, and its neighboring experiment, ATHENA, both use antiprotons from CERN's Antiproton Decelerator to produce cold antihydrogen. ATHENA uses more positrons, and deduces the existence of cold antihydrogen atoms from observations of the simultaneous annihilations of antiprotons and positrons when antihydrogen atoms annihilate upon hitting matter. ATRAP provides the first glimpse inside antimatter atoms, observes cold antihydrogen atoms with no noise background at all, and observes more antihydrogen atoms than ever before. Both teams accumulate extremely cold antiprotons using techniques that were developed by ATRAP and its predecessor. Both also use a nested Penning trap, a device developed by ATRAP scientists to allow the gentle collisions of antiprotons and positrons needed to form cold antihydrogen

Given the strong start, the future for precise studies of antihydrogen now seems bright at CERN. ATRAP scientists caution that they still have many experiments to do, much apparatus to design, many techniques to invent, many student to train, and many night shifts to work before there is a precise comparison of antihydrogen and hydrogen. Encouraged by the success they are eager to move forward.

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Release is Based on a Scientific Publication by ATRAP:
"Background-Free Observation of Cold Antihydrogen with Field-Ionization Analysis of Its States"
G. Gabrielse, N.S. Bowden, P. Oxley, A. Speck, C.H. Storry, J.N. Tan, M. Wessels, D. Grzonka, W. Oelert, G. Schepers, T. Sefzick, J. Walz, H. Pittner, T.W. Hänsch, E.A. Hessels
Physical Review Letters (in press)

ATRAP Spokesperson: Professor Gerald Gabrielse

Collaborating Institutions and Contact Persons
Harvard University (Cambridge, MA, USA)
Professor Gerald Gabrielse
Chair of the Physics Department
gabrielse@physics.harvard.edu

Forschungszentrum Jülich (Jülich, Germany)
Professor Walter Oelert
Forschungszentrum Jülich, IKP
52425 Jülich, Germany Jülich:

Max-Planck-Institut für Quantenoptik (Garching, Germany)
and Ludwig-Maximilians-Universität (München, Germany)
Dr. Jochen Walz (CERN-Fellow 2001--2002)
Hans-Kopfermann-Str. 1 Jochen.walz@mpq.mpg.de 85748 Garching, Germany

York University (Toronto, Canada)
Professor Eric Hessels
Dept. of Physics and Astronomy
hessels@yorku.ca
Petrie Science Building York:
Toronto, Ontario M3J 1P3, Canada

Background Information
Accumulating cold antiprotons
Old popular treatment: "Extremely Cold Antiprotons"
G. Gabrielse;
Scientific American, December, 1992 p. 78-89.

A general review:
"Comparing the Antiproton and Proton, and Opening the Way to Cold Antihydrogen"
G. Gabrielse
in Advances in Atomic, Molecular, and Optical Physics, Vol. 45, edited by B. Bederson and H. Walther,
Academic Press, New York, pp. 1-39 (2001).

Recent research paper: ''Stacking of Cold Antiprotons''
ATRAP Team
Phys. Lett. B 548, 140 (2002)

Accumulation of cold positrons
Research paper: "Field ionization of Strongly Magnetized Rydberg Positronium: A New Physical Mechanism for Positron Accumulation",
J. Estrada, T. Roach, J.N. Tan, P. Yesley and G. Gabrielse;
Phys. Rev. Lett. 84, 859 (2000)

Nested penning trap
Proposal: "Antihydrogen Production Using Trapped Plasmas"
G. Gabrielse, L. Haarsma, S. Rolston and W. Kells
Physics Letters A 129, 38 (1988)
Used with antiprotons and positrons: "The Ingredients of Cold Antihydrogen: Simultaneous Confinement of Antiprotons and Positrons at 4K",
G. Gabrielse, D.S. Hall, T. Roach, P. Yesley, A. Khabbaz, J. Estrada, C. Heimann, and H. Kalinowsky;
Phys. Lett. B 455, 311 (1999)

First positron-cooling of antiprotons (uses nested Penning trap)
Research paper: ''First Positron Cooling of Antiprotons''
ATRAP Team
Phys. Lett. B 507, 1 (2001)

Report that antihydrogen is formed during positron-cooling of antiprotons in a nested Penning trap Research paper: "Production and Detection of Cold Antihydrogen"
ATHENA Team
Nature 419, 456 (2002)

First observation of high velocity antihydrogen
Research paper: "Production of Antihydrogen"
G.Baur et al.
Phys. Lett. B 368 (1996) 251-258

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